The invention relates to solar cells in general and particularly to a dye-sensitized solar cell that employs zinc oxide aggregates grown in the presence of deliberately added lithium ions.
The worldwide demand for energy has increased, and the consumption of oil reserves raises the possibility that certain forms of fossil energy may have reached their peak in production. This has spurred the development of new energy sources that are cost-effective and environmentally-friendly. Solar radiation is one source of energy that potentially has no negative environmental impact. The conversion of solar radiation to electricity is accomplished using solar cells. These devices generate electrical carriers through the photovoltaic effect and then separate the photogenerated carriers to provide current at an operating voltage or electrical potential. For several decades, crystalline silicon and compound semiconductor thin films have been developed for solar cell use. However, they possess the disadvantage of high production costs. Interest has developed in dye-sensitized solar cells (DSSCs) as a consequence of their low cost and relatively high conversion efficiencies.
Many wide band gap oxides such as TiO2, SnO2 and Nb2O5, have been investigated as photoelectrode materials in DSSCs. Nanostructures such as nanoparticles, nanowires/nanorods, and nanotubes offer a large surface area for dye adsorption and/or a direct pathway for electron transport. As reported by J. M. Kroon, et al., in “Nanocrystalline Dye-Sensitized Solar Cells Having Maximum Performance,” Progress in Photovoltaics 15(1):1-18, 2007; and M. Gratzel in “Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells,” Inorganic Chemistry 44(20):6841-6851, 2005, to date a maximum solar-to-electricity conversion efficiency of about 11% has been obtained with TiO2 nanocrystalline films
In addition to having the desired photoelectrode film structure, the achievement of good conversion efficiencies for DSSCs is also attributed to the use of ruthenium-based dyes as the photo-sensitizer. These dyes, known as N3, N719, or black dye, are efficient in capturing photons with wavelengths in the visible region. More importantly, the photogenerated electrons in these dyes have long excited-state lifetimes (˜100 fs) and, therefore, can be effectively injected from the dye molecules into the semiconductor before radiative or non-radiative recombination occurs (˜15 ns).
Zinc oxide (ZnO) also has been regarded as a candidate in DSSCs. The use of ZnO is thought to be advantageous at least in that 1) it can easily be fabricated into various nanostructures, and 2) it possesses a high electron mobility. However, the reported conversion efficiencies are still relatively low. Conversion efficiencies of 1.5˜5% for ZnO nanocrystalline films, 0.3˜4.7% for ZnO nanowires, 1.6˜2.3% for ZnO nanotubes, and 0.23˜5.08% for ZnO nanoporous films have previously been attained.
One issue that is a concern in ZnO-based DSSCs is the inability to obtain sufficient specific surface area for ZnO films. In addition, the poor photovoltaic performance of ZnO-based DSSCs may be caused by the instability of ZnO in ruthenium-based dye solutions. The immersion of ZnO in ruthenium-based dyes results in the formation of an inactive Zn2+/dye complex layer on the ZnO surface. This layer serves to lower the injection efficiency of electrons from the dye molecules into the ZnO semiconductor.
There is a need for a ZnO DSSC that addresses the deficiencies of the prior art yielding a solar cell with higher conversion efficiencies.